Damage Mechanisms Found in Generator Rotor 18Mn18Cr Retaining Rings

Abstract

Generator rotor retaining rings are one of the most highly stressed components in the generator rotor. Additionally, some retaining ring materials such as 18Mn5Cr, have been susceptible to Stress Corrosion Cracking (SCC). 18Mn18Cr retaining ring material has been used to replace older 18Mn5Cr material rings with great success. The 18Mn18Cr material has been found to be resistant to SCC in the presence of moisture. Recently, one OEM (Original Equipment Manufacturer) called for inspections of 18Mn18Cr rings, despite its reliable performance in the industry. Although resistant to SCC, some 18Mn18Cr rings have been found with cracks and other damage. The author’s company felt that it would be of value to go back through past job records with rotors that were identified to have 18Mn18Cr retaining rings, and review those records and report on the results of ring inspections due to damage. The author’s company typically rewinds dozens of rotors per year, with many of the rings manufactured from 18Mn18Cr. As part of a rewind, rings are disassembled and inspected. Of course, many rewinds are done because of failures, mostly related to the field winding. An XRF (X-Ray Fluorescence) analyzer is used to determine a ring’s composition. It has excellent accuracy, and can easily distinguish between 18Mn18Cr material and other non-magnetic steels, such as the once common 18Mn5Cr. Magnetic rings are easily distinguishable by a magnet, by manufacturer and rating, and / or by ventilation method. A total of seventeen (17) 18Mn18Cr ring metallurgical investigation reports were reviewed. These metallurgical investigations were conducted between 2008 and 2014. Reports reviewed included only those retaining ring inspections that were done to diagnose some visible damage to one or two retaining rings. In other words, if an 18Mn18Cr retaining had some indication of damage, the detailed inspection report and analysis was included and reviewed as part of this paper. Rings that were inspected and had no issues are not included. Rings that had damage due to some obvious man-made issues were not included. For example, one generator rotor had an 18Mn18Cr ring that had an obvious grinding wheel cut. The owner was aware that this damage had occurred during a prior repair. This metallurgical investigation was not included in this study. Another finding not included in this study was an 18Mn18Cr retaining ring with a high hardness variation, well beyond the tolerance standards and acceptance criteria used by NEC. Since there was no physical damage to the ring surface and no cracking, pitting or other abnormality present, this ring’s inspection report was also excluded. Only rings with some type of visual evidence of pitting, corrosion, fretting or material upset were included and make up the 17 generator rotors.